From the perspective of a base station, such as an evolved node B (eNB) in an evolved universal terrestrial radio access network (E-UTRAN) system, the signals from all the mobile terminals should arrive at the same time. The E-UTRAN system enables this by using a timing advance (TA) to control timing of the uplink (UL) transmissions from the mobile terminals. This TA also compensates for delay in the signal propagating from the sending mobile terminal to the receiving base station. Specifically, third generation partnership project (3GPP) TS 36.211 v10.0.0 (2010-12) from which FIG. 1 is taken sets forth that transmission of the uplink radio frame number i from the mobile station shall start (NTA+NTA offset)×Ts seconds before the start of the corresponding downlink radio frame at the mobile terminal, where 0≦NTA≦20512, NTA offset=0 for frame structure type 1 and NTA offset=624 for frame structure type 2. Note that not all slots in a radio frame may be transmitted; for example in the time division duplex (TDD) mode only a subset of the slots in a radio frame are transmitted.
The UL and the downlink (DL) transmissions between the same base station (or radio head/repeater/relay) and the mobile terminal have the same propagation path and speed. So from the point of view of the base station, the base station will control the time at which it receives the UL transmission so as to align with the DL transmission timing. Therefore, the timing difference between a DL transmission sent by the base station and an UL transmission sent by the mobile terminal should be the same as the difference between the DL reception at the mobile terminal and DL transmission from the base station. This relationship is shown graphically at FIG. 2, where the timing values refer to the time at which the relevant transmission is sent or received. The difference [TUT−TDT] between the time TDT at which the base station sends the DL transmission and the time TUT at which the mobile terminal sends its UL transmission is the same as the difference [TDT−TDR] between the time TDT and the time TDR at which mobile terminal receives the DL transmission from the base station. The timing advance is the round trip time, and equation (1) below expresses the TA and the equivalence of the timing differences explained above.TA=TDR−TUT TDT−TUT=TDR−TDT  (1)From equation (1), it follows that TA=2*(TDR−TDT).
The utilization of TA to synchronize the receipt of UL transmissions by a base station may be more complex in a system that employs carrier aggregation (CA). In this regard, wireless systems, such as the Long Term Evolution (LTE)-Advanced wireless system, aims to provide enhanced services by means of higher data rates and lower latency with reduced cost. CA is one technology that LTE-Advanced systems intend to employ for improving the data rate. FIG. 3 illustrates the CA concept. As shown, the entire bandwidth of the wireless system is divided into two or more component carriers (CCs), of which FIG. 3 shows five CCs by example. At least one CC is configured to serve legacy mobile terminals. Release 10 and later mobile terminals are to be capable of monitoring/using multiple CCs, and so the wireless network is able to assign two or more CCs simultaneously as active for a single mobile terminal. This enables the network greater scheduling flexibility by giving it the ability to allocate channels to the same mobile terminal on any one or more of the multiple CCs assigned to a given mobile terminal. In the case multiple CCs are assigned and active for a mobile terminal, one of the assigned CCs will be the mobile terminal's primary CC and the other(s) will be secondary CC(s). The mobile terminal's secondary CC(s) is/are also sometimes termed an extension carrier.
For Release 10, 3GPP has agreed that there will be only intra-band CA for the UL and one TA for all the UL CCs. But in Release 11 and beyond, when taking inter-band CA into deployment, as well as the cases of radio remote head (RRH) and repeaters (which are conceptually similar to relay stations for the purposes herein), multiple TAs will be necessary.
When CA is introduced, it may be that not all CCs assigned for the mobile terminal are on the same timing, and so the TA on one CC is not valid for another CC on which the mobile terminal is communicating simultaneously. In this case the mobile terminal will need to adjust the UL transmission timing on the second CC in order to assure its UL transmissions are synchronized for the base station (or other reception node such as a repeater).
If one of the CCs is termed the mobile terminal's primary cell (PCell), and the other asynchronous CC is termed the mobile terminal's secondary cell (SCell), then the timing relation shown by example at FIG. 4 graphically illustrates the multiple timing advances. Note that in FIG. 4, the UL and DL radio frames on the mobile terminal's PCell are the same as those shown in FIG. 2; but FIG. 4 shows also the similar radio frames transmitted on the mobile terminal's SCell which are asynchronous with those on the PCell.
The TA for each of the CCs may be determined in various manners. By way of background, for Releases 8/9/10, the only way for a mobile terminal which was not yet synchronized with a serving base station to measure the timing advance was by accessing the random access channel (RACH). For Release 10, it was also agreed that random access will only be performed on the mobile terminal's primary CC, and so the mobile terminal was not required to know the RACH configuration on any secondary CCs.
For Release 11 and beyond, when multiple TAs are introduced, a mobile terminal also had to determine the TA value for its SCell(s). Simply requiring the mobile terminal to utilize the RACH procedure to learn the TA on an SCell would require that the RACH configuration on the SCell be indicated to the UE somehow, and also this would lead to some changes to the current SCell parameter structure.
Additionally, in Release 10, RACH failure is recognized as a trigger condition for radio link failure (RLF). This followed from RACH being performed only on the PCell, but if the mobile terminal hypothetically also had RACH access on the SCell, there would be a need for further standardization as to what would be a trigger to indicate UL RLF. These more nuanced issues are in addition to the straightforward ones, such as if the mobile terminal is to get the SCell TA on an SCell RACH, there would necessarily be an increase to RACH overhead, meaning a higher load on the SCell RACH due to a higher number of mobile terminals accessing it and also greater potential for delay on the RACH since more mobile terminals would be competing for a slot on it.
As described in International Patent Application No. PCT/CN2011/070874 filed on Feb. 1, 2011 entitled Timing Advance Without Random Access Channel Access, the contents of which are incorporated herein by reference, a mobile terminal may determine the TA of a secondary cell based upon the timing difference between the primary cell and the secondary cell. By determining the TA of a secondary cell based upon the timing difference between the secondary cell and the primary cell, the TA of the secondary cell may be determined without access to the RACH of the secondary cell and without the issues potentially created by access to the RACH of the secondary cell.
In conjunction with the establishment of a TA for any of the CCs, a timer may be maintained which defines the period of time during which the TA remains valid for the respective CC. With CA and the establishment of different TAs for a number of CCs, however, the mobile terminal may have to establish and maintain timers for the TAs of each of the CCs, which may become more complex with the multiplicity of CCs.